Sound absorptive structural block



April 14, 1970 C. A, U N,GER 3,506,089

'4 scum) ABSORPTIVEV STRUCTURAL BLOCK Filed Oct. 25, 1968 r 1/ \\l I I Il r i i s 20 i L 32' I l l I! ll 1 INVENTOR. MIGUEL C. JUNGER ATTORNEYSUnited States Patent US. Cl. 181-33 3 Claims ABSTRACT OF THE DISCLOSUREA sound absorptive block of molded structural material has a cavity orcavities in communication with the sound source through an elongatedslot or slots flared in width from the cavity toward the outer surfaceof the block. Within a certain frequency range, the air in the cavityreacts resonantly to impinging sound Waves to cause significant airmovements within the slot and consequent dissipation of sound energythrough turbulence and friction losses. The flare in the slot increasesthe aperture areaexposed to the sound. It also provides an impedancematching effect that increases the transmission of impinging soundenergy into the cavity and its dissipation therein at frequencies abovethe resonant frequency.

BACKGROUND OF THE INVENTION The US. Patent to O. Zaldastani and thepresent applicant No. 2,933,146 discloses a sound absorbing block ofmolded structural material such as concrete. This block has one or morecavities communicating with a source of impinging noise through one ormore slots, whereby sound energy is dissipated by a combination ofseveral effects. These include a so-called Helmholtz resonance effectresulting in the dissipation of sound energy within the walls of theslot and a black body effect resulting in the dissipation of soundenergy by multiple reflections within the cavity.

The usefulness of this block as a sound absorber is a function of theaperture area of the slot exposed to impinging sound and of the acousticimpedance of the cavity as viewed from the source of impinging sound.The acoustic impedance of the cavity at low frequencies is spring-likeand large, at frequencies near the Helmholtz resonance it is resistiveand small, and at frequencies above the Helmholtz resonance it ismass-like and large. In practice the conditions are such that theHelmholtz resonance occurs between and 300 Hz., approximately, and it isrelatively unimportant to dissipate sound energy at frequencies belowthis range but of substantial practical importance to dissipate soundenergy at frequencies above this range.

The block described in said patent provides effective sound dissipationat frequencies near the Helmholtz resonance. Here, the impedance of thecavity is well matched to that which characterizes the incident soundwaves, namely a relatively small, purely resistive impedance that iseffectively independent of frequency. As noted above, the impedancemismatch that occurs at lower frequencies is not disadvantageous.However, a limitation on the sound absorbing performance of the blockexists at middle and higher frequencies above the Helmholtz resonancewhere an impedance mismatch limits the transmission of sound energythrough the slot into the cavity.

It is a principal object of this invention to provide an improved blockcapable of greater sound absorption at frequencies above the Helmholtzresonance.

Additional objects are to achieve the desired sound absorption byeconomical and reliable means and without sacrifice of the variousadvantages attributable to the block described in said patent, and toachieve an aesthetiice cally pleasing appearance in applications wherepartitions formed of the blocks are significant factors in architecturaldesign.

SUMMARY OF THE INVENTION This invention consists in a masonry structurehaving a slot or slots of improved configuration that enhances the soundtransmission to an enclosed cavity or cavities, whereby a greaterdissipation of sound energy is achieved. The configuration, referred togenerally as flared, is one of variable width increasing from theextremity of the slot adjacent the cavity toward the opposite or outerextremity. This enables the slot aperture to comprise a larger fractionof the surface of the structure exposed to sound, as compared with slotsof uniform width equal to that adjacent the cavity. It also results inan improved absorption cross section of the cavity whereby a highertransmission ratio is achieved, thus leading to greater sound absorptionby the slotted structure. This higher transmission ratio is achieved bymatching the acoustic impedance of the cavity to the impedance whichcharacterizes the incident sound waves.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a front elevation of amasonry block embodying the invention.

FIG. 2 is a bottom view of the block shown in FIG. 1.

FIG. 3 is an elevation in section taken on line 3-3 of FIG. 1.

FIG. 4 is an isometric view of a mold plug used in casting the block ofFIGS. 1 to 3.

FIG. 5 is a fragmentary side elevation of the mold plug.

DESCRIPTION OF THE PREFERRED EMBODIMENT A masonry block 12 according tothis invention is shown in FIGS. 1 to 3. It is manufactured in the usualmanner by means of a block mold adapted to pack the hardenable aggregatearound a pair of tapered plugs 14, the latter having means to form theslots. The masonry material may be any hardenable type such as concreteor the like.

After compacting the material, the hold is stripped, the block is cured,and when hardened is a load-bearing structural element having cavities16 and 18. The block has a pair of closed end walls 20, a third or topclosed wall 22 contiguous with the walls 20, a fourth or back closedwall 24 contiguous with the walls 20 and 22, a closed partition wall 26(or plural partition walls if there are more than two cavities), and afifth or front wall 28 opposite the fourth wall and intended to face thesource of sound to be suppressed.

The wall 28 has apertures in the form of flared slots 30 and 32 by whichthe cavities 16 and 18 are in communication with air propagating thesound to be suppressed. The Width of these slots increases uniformlyfrom their extremities adjacent the cavities to the opposite or outerextremities as hereinafter further explained. This shape of the slots ispreferably produced in the fold by means of inserts 34 received in slotsin the plugs 14, the inserts being shaped in the desired manner.Alternatively, the plugs may have integral fins shaped like the inserts34.

Various flared shapes may be used according to this invention. Astraight line or V-shaped taper, that is, one in which the width of theslots varies linearly between its extremities, is one form. Thepreferred form is an exponential taper, that is, one in which the slotwidth varies approximately as a constant raised to a power which'is alinear function of the position between the slot extremities. Thisresults in smooth slot walls that are free of sharp exposed outercorners or edges and of pleasing appearance, as viewed in FIG. 1. Italso results in better impedance matching, as more fully explainedbelow.

The blocks 12 are laid up in successive courses with mortar to formpartition walls or ceilings. Thus one course closes the cavities 16 and18 of an adjacent course except for the slots, whether or not the jointsbetween the side walls 20 are offset between successive courses.

As thus laid up in a masonry structure, the blocks are effective toabsorb and dissipate the sound impinging on the wall 28. Dissipationresults in part from multiple reflections within the cavities 16 and 18,which is the socalled black body effect. Dissipation also results fromlosses within the slots 30 and 32 that are enhanced at frequencies forwhich there is acoustical resonance, with sound waves of maximumamplitude oscillating within the slots. This phenomenon is associatedwith the Helmholtz resonance effect.

It is evident that the flaring of the slots increases the fraction ofincident acoustic energy that is transmitted into the cavity where it isabsorbed. In physics terminology, the absorption cross section of thecavity is increased.

Also, the flared slots match the impedance of surrounding air to that ofthe cavities in the manner of an ear trumpet used by the hard ofhearing. The result is a more effective sound absorptive structure. Thiswill be evident from a consideration of the theory of acoustical horns,as presented for example in H. F. Olson, Elements of AcousticalEngineering, 2nd ed., D. Van Nostrand Co. (New York, 1947), pp. 94l12.Although textbooks generally deal with horns as devices coupling ahigh-impedance source to the atmosphere, whereby acoustic energy flowsfrom the throat to the mouth of the horn, these analyses are, by virtueof the reciprocity principle, equally applicable to energy flow in thereverse direction, whereby energy flows from the mouth to the throat.The effect of the flared shape is such that in the medium frequencyrange above the Helmholtz resonance, the transformation ratio is smalland a phase shift is effected from a reactive impedance at the throat toa resistive impedance at the mouth, that is, an improved impedance matchas compared with the blocks described in said patent. At lowfrequencies, i.e., near and below the Helmholtz resonance, thetransformation ratio is unity and the sound absorption is comparable tothat achieved by the blocks described in said patent. A mismatch ofimpedances occurs at frequencies below the Helmholtz resonance, but thisis of no practical disadvantage.

As previously noted, the preferred shape of slots is exponential. Thisproduces a larger ratio of resistance to reactance, that is, betterimpedance matching than a straight sided taper, although the latter aswell as other flared shapes can be used in many applications.

Comparing a cavity having a slot of uniform width w with one having aflared slot with a minimum width equal to w adjacent the cavity, it isfound that the latter has a higher natural or resonant frequency. Thismay be seen by considering the general formula for the natural orresonant frequency f of a Helmholtz resonator.

in which k is the mechanical stiffness of the cavity and constant inthis comparison, and m is proportional to the kinetic energy of the massof air in the slot, augmented by the accession to inertia or entrainedmass of air. For a slotted masonry block in which the thickness t of thewall 28 is significantly larger than the width of the slot, theaccession to inertia is small compared to the mass of air actuallylocated in the slot, and may be ignored. From this expression, it isseen that the ratio of the natural frequency f for the flared slot tothe natural frequency f for the uniform slot is given by in which T andT are the corresponding kinetic energies 4 for equal air velocities v atthe throat. The kinetic energy of the air in the slot depends on itsshape. For an exponentially flared slot, the slot width w at anyposition x measured from the cavity toward the outer extremity is givenby in which a is the flare constant. The air velocity v at the positionx is given by This expression gives recognition to the fact that the airis substantially incompressible within the range of frequencies ofinterest, and therefore the mass flow rate is a constant at every crosssection of the slot.

The kinetic energy T for the uniform slot may be expressed :by using theformula for kinetic energy which applies when the velocity is the sameat all the cross sections.

(5 T /2 Lw tv in which is the density of air, and L the vertical slotlength. By similar means and using Equations 3 and 4, the expression Tfor the flared slot is obtained.

6 i2 1 at T =l/2pLw v fl edx= T,,( Using Equation 6, Equation 2 may berewritten.

cavity. Thus, (9) )1/2 i/2 with f,, expressed in Hertz and length ininches. By Equations 7 and 9,

The ratio (7) is always larger than one; hence the natural frequency forthe flared slot always exceeds that for a uniform slot. If the aboveanalysis is refined to account for the accession to inertia, the ratioin Equation 7 is increased.

From the foregoing description it will be evident that each block mayhave one or more cavities. Also, although the drawing shows the slots 30and 32 as extending the full height to the wall 22, one or both of theseslots may be shorter in length, if desired. Further, although the plug14 of FIGS. 4 and 5 is adapted to produce a slot with its end surfacetapered, it may be modified to produce a slot with a flat end surface,either at right angles or at an acute angle to the front wall 28. As astill further modification, sound absorptive material such as a body 36of fibrous material may be inserted in the blocks in such a position asto extend across the inner end of the slot. This material prevents somekinds of foreign matter from eing inserted in the blocks and assists indamp-ing the sound and dissipating its energy. Other variations in thestructure will occur to a person skilled in this art, and

will also fall within the spirit and scope of this invention.

I claim:

1. A sound absorptive block of molded structural material having acavity, a pair of opposite closed end walls, third and fourth contiguousclosed outer walls each contiguous with each of the end walls, a sidehaving an opening opposite the third Wall and extending to the cavity,and a fifth outer wall opposite the fourth wall, having an 5 6 exposedsurface facing the direction of noise to be ab- 2,007,130 7/1935 Munroeet al. sorbed and having a slot therein leading to the cavity, 2,293,1818/1942 Terman. said slot being elongated, having one end terminating at2,362,859 11/1944 Rosenblatt.

said opening, extending in length from the opening toward 2,840,179 6/1958 Junger. the third wall, and being of variable width increasing2,933,146 4/1960 Zaldastani et al.

from its extremity adjacent the cavity to said exposed 5 3,275,1019/1966 Morrissey et al. surface.

2. A block according to claim 1, in which the walls of FOREIGN PATENTSthe slot are smoothly flared toward the exposed surface 181,414 3/1955Austria of the fifth Outer k 10 606,872 10/1960 Canada.

3. A block according to claim 2, in which the walls 889,063 9 1 53Germany of the slot are flared substantially exponentially toward 15 1026/1959 Great Britain the exposed surface of the fifth outer wall.959,117 5 19 4 Great Britain 1 1 3 References Cited 15 3 2 4/1951 SwedenUNITED STATES PATENTS ROBERT S. WARD, JR., Primary Examiner 625,8695/1899 Buckley. US. Cl. X.R. 1,554,180 9/1925 Trader. 52 145 2,002,5105/1935 Rosenblatt. 20

Paient I-Eo 3 506 089 Dated April 14, 1970 I1went0 r(s) Migul C. JungerIt is certified that error appcar in the above-identificd patent ;;-L Hl LEifiiiili95$?EFLRQR RRE 'Q. corrected as Shown below:

